Bugs in python code in README

[bjornaa]

Hi.

This is a minor issue, but the python script in the README has two errors.
These errors do not affect the conversion, it is just to make the README look nicer.

Here is a diff between the README code and a corrected version.

 18c18
 < import matplotlib.pyplot all_plt
 ---
 > import matplotlib.pyplot as plt
 22c22
 < plt(x,y)
 ---
 > plt.plot(x,y)